Apparatus for measuring the degree of delocculation of a suspension of fine particles

ABSTRACT

An apparatus and process for measuring and controlling the degree of deflocculation of a suspension of fine particles. The apparatus includes a centrifuge having two separate compartments into one of which there is introduced a portion of the suspension under examination and into the other of which there is introduced a further portion of the suspension under examination which has been treated with excess deflocculant so as to completely deflocculate said further portion. The compartments of the centrifuge are each provided with a window whereby light can be passed through the portions of the suspension thereunder. Photocells are employed to measure the optical densities, and thus the solids concentrations, of the parts of the suspension in the compartments through which the light is passed and this enables the degree of deflocculation of the suspension to be ascertained.

United States Patent Inventors Appl. No.

Filed Patented Assignee Priority APPARATUS AND METHOD FOR MEASURING THEDEGREE OF DEFLOCCULATION OF A SUSPENSION OF FINE PARTICLES 9 Claims, 4Drawing Figs.

US. Cl 356/36, 73/6l.4, 73/432, 250/218, 356/72 Int. Cl G01n1/00, GOlnl5/04,G01n 21/00 Field of Search 356/36, 72;

250/218; 73/61.4, 432 (PS), (Inquired); 210/54;

Primary Examiner-James W. Lawrence Assistant Examiner-T. N. GrigsbyAttorney-Larson, Taylor and Hinds ABSTRACT: An apparatus and process formeasuring and controlling the degree of deflocculation of a suspensionof fine particles. The apparatus includes a centrifuge having twoseparate compartments into one of which there is introduced a portion ofthe suspension under examination and into the other of which there isintroduced a further portion of the suspension under examination whichhas been treated with excess deflocculant so as to completelydeflocculate said further portion. The compartments of the centrifugeare each provided with a window whereby light can be passed through theportions of the suspension thereunder. Photocells are employed tomeasure the optical densities, and thus the solids concentrations, ofthe parts of the suspension in the compartments through which the lightis passed and this enables the degree of deflocculation of thesuspension to be ascertained.

DETERMINE- 77/6' 056/956 OF JE/Z OCCULAT/O/V 0F 77/5 09/5/11/4L 51/5 PEN3 0N Patented March 30, 1971 3 Sheets-Sheet 1 l I a j 7 4 w L. w M w/L\ir {M Patented March 30, 1971 3 Sheets-Sheet 2 m UE mw ukkw Qwk I x86APPARATUS AND METHOD FOR MEASURING THE DEGREE OF DEFLOCCULATION OF ASUSPENSION OF FINE PARTICLES BACKGROUND OF THE INVENTION This inventionrelates to an apparatus and process for measuring, and optionallycontrolling, the degree of deflocculation of a suspension of fineparticles, for example a clay slip.

Whenever a suspension of fine particles, i.e. particles substantiallysmaller than 100 microns, is subjected to a particle size classificationprocess, it is common practice to deflocculate the suspension prior tothe particle size classification process. Deflocculation is aphysico-chemical process in which, by the absorption of suitably chargedions, the fine particles become equally charged giving rise to repulsiveforces between the particles, thus overcoming the attractive Van derWaals forces and allowing the particles to move independently and not asa cohesive floc of particles of variable size. Commonly useddeflocculants are sodium silicate, sodium polyphosphates and sodiumsalts of poly-(acrylic acid). The usual method of controlling theaddition of deflocculant is by control of pH. This has the disadvantagethat the pH at which a suspension of fine particles reaches a givendegree of deflocculation is not an invariable but depends upon severalfactors such as the type of mineral and its previous chemical treatment.It is inevitable, therefore, that with the pH control set to cover alleventualities, the addition of deflocculant will often be greater thanthat really required. Also, the method is not suitable for use withdeflocculants which do not cause a change in pH.

It is an object of the present invention to provide a process and anapparatus which will enable the degree of deflocculation of a suspensionof fine particles to be ascertained.

SUMMARY OF THE INVENTION It is found that when a suspension of fineparticles sediments for a given time, the solids concentration of thesuspension at a point a little way below the surface of the suspensionis dependent on the degree of deflocculation. By the term solidsconcentration" there is meant herein the weight of solids in a givenvolume of the suspension. The greater the degree of deflocculation, i.e.the more deflocculated the suspension, the greater will be the number offiner particles present, which settle more slowly and therefore thehigher the finer particle concentration in the upper levels.

If a quantity of a suspension of fine particles, of which it is desiredto determine the degree of deflocculation, is divided into two portionsand an additional quantity of deflocculant is added to one portionsufficient to cause complete deflocculation, then a comparison of thesolids concentrations of the two portions of the suspension at identicaldepths and after identical sedimentation times will give a measure ofthe degree of deflocculation of the original sample, the other factors,such as initial solids concentration and particle size distribution,having been eliminated.

It is therefore a further object of the present invention to provide aprocess and an apparatus which will enable there to be carried outrapidly a comparison of the solids concentrations of suspensions of fineparticles at identical depths and after identical sedimentation times.

Accordingly, in one aspect the present invention provides an apparatuscomprising a centrifuge having two separate compartments each providedwith a window, whereby light can be passed through material present insaid compartment, and a photocell arranged so as to interrupt the lightpassing through said window and the material in said compartment and toproduce a signal corresponding to the optical density of materialpresent in said compartment at a fixed distance from the axis ofrotation of said centrifuge, and means for comparing the signalsobtained from each compartment.

According to another aspect of the invention there is provided a processfor measuring the degree of deflocculation of a suspension of fineparticles which process comprises separating said suspension into twoportions, adding to one of said two portions a quantity of adeflocculant sufficient to completely deflocculate said one portion,introducing the portions of said suspension one into each of twoseparate compartments of a centrifuge, said two compartments each beingprovided with a window, whereby light can be passed through the portionof said suspension therein, and each having associated therewith aphotocell arranged so as to interrupt the light passing through saidwindow and the suspension in said compartment and to produce a signalcorresponding to the optical density of the portion of said suspensiontherein at a fixed distance from the axis of rotation of saidcentrifuge, centrifuging said two portions for a time dependent on theinitial solids concentration of the suspension, passing light throughsaid windows and through said portions of the suspension, and measuringthe optical densities of the two portions at said fixed distance fromthe axis of rotation of said centrifuge, whereby the degree ofdeflocculation of the original suspension can be determined.

As the rate of sedimentation of the solid particles in a suspensiondepends on the diameters of the particles and the difference between thespecific gravit'ies of solids and liquids, a sedimentation time ischosen so that the particles left in suspension are the ones whosedegree of deflocculation it is particularly desired to measure. Forexample, in the case of china clay we use a centrifuging time whichleaves in suspension particles of about 1 micron equivalent sphericaldiameter and smaller. Fine particles require more deflocculant per unitweight than coarse particles so it can be assumed that if the finestparticles are deflocculated all will be. If, for example, the solidsmaterial is a crushed metallic ore having only about 20 percent byweight of particles smaller than 10 microns, the centrifuging time mightbe chosen so that all particles greater than about 10 microns aresedimented.

The apparatus of the invention enables there to be obtained a numericalvalue, which we shall designate the deflocculation ratio, whichincreases as the suspension becomes more deflocculated. Thisdeflocculation ratio is determined by the ratio of the solidsconcentration of the sample whose degree of deflocculation it is desiredto determine to the solids concentration of the fully deflocculatedsample at a centrifuging time and at a depth below the free surfacechosen to give particles of the desired size in suspension at that timeand at that depth. In order that the results may be self consistent,i.e. in order that a higher deflocculation ratio is invariably assignedto a more deflocculated suspension, slight adjustments are made to thecentrifuging time dependent on the initial solids concentration of thesuspension since the ratio of photocell resistances is not constant fora given ratio of solids concentrations but depends on the initial solidsconcentration of the suspension.

In a preferred embodiment of the apparatus of the invention, thecentrifuge comprises two cooperating disc members arranged face-to-face,at least one of which is formed with recesses which constitute theseparate compartments of the centrifuge. Advantageously, there areprovided two truncated hollow cones arranged one within the other andcoaxially with the axis of rotation of the centrifuge, one truncatedhollow cone being in communication with one of said separatecompartments and the other truncated hollow cone being in communicationwith the other of said separate compartments, the arrangement being suchthat said compartments can be filled with suspensions of fine particlesduring operation of the centrifuge. It is also advantageous if there isprovided means which enable the discs to be separated whilst thecentrifuge is in operation whereby the compartments can be emptied.

DESCRIPTION OF THE PREFERRED EMBODIMENTS For a better understanding ofthe invention and to show how the same may be carried into effect,reference will now be made, by way of example, to the accompanyingdrawings, in which:

FIG. 1 is a plan view ofa centrifuge for use in an apparatus accordingto the present invention,

FIG. 2 is a sectional elevation of the centrifuge shown in FIG. 1 takenalong the line II-Il of FIG. 1, and

FIG. 3 is a block diagram showing schematically one embodiment of theapparatus of the present invention.

Referring first to FIGS. 1 and 2, there is shown a centrifuge whichcomprises two separate sample compartments A and B. The samplecompartments are defined by two discs 1 and 2, the upper disc 1 beingrecessed a depth of one-eighth inch to form the compartments. Except forthe innermost side, the compartments are sealed by rubber sealing strips9. The discs 1 and 2, and thus the compartments A and B, are rotatedabout a central axis by means of a hollow drive shaft 3. Thecompartments A and B can be filled, whilst being rotated, throughtruncated hollow cones 4 and 5 arranged one within the other. Slot 18permits passage of material from the outer truncated hollow cone 4 tosample compartment B. Slot 19 in the wall of the outer truncated hollowcone 4 represents the exit of the tunnel from inner truncated hollowcone 5. Numeral 20 represents the end of the tunnel of square crosssection which conveys material from the inner truncated cone of thesample compartment A. Each sample compartment is provided with glasswindows 6 and 7 which are fitted in the upper and lower discs 1 and 2,respectively, at the same radial distances from the axis of rotation ofthe discs 1 and 2 i.e. with the centers of the windows five-eighths inchfrom the free surfaces of the suspensions, the windows in eachcompartment being separated by equal distances. Pairs of photoconductivecells 8 are mounted directly below the lower windows 7 in eachcompartment, and electrical connection to the photocells 8 is made by aslip ring assembly 14, the photocells 8 being connected into aself-balancing Wheatstone bridge network. A ring of lamps (not shown)are disposed around the truncated cones 4 and 5 and arranged so thatlight can be directed continuously from the lamps through the windows 6and 7 and through the material in each compartment onto the photocells 8during centrifuging. An overflow system comprising an overflow path Pand a levelling hole 13 ensures that the compartments A and B are filledto identical levels. The upper disc 1 is connected by pillars 15 to anannular member 16 mounted below the lower disc 2. The annular member 16supports eight symmetrically disposed hydraulic cylinders 10 eachcontaining a piston 17 which urges the two discs together. Hydraulicfluid is introduced through the bore of the shaft 3 and passes throughholes to the outer end of each of which is connected, by a compressionjoint, a tube leading to one of the hydraulic cylinders 10. When the oilpressure in the hydraulic cylinders 10 is released the discs separateunder the action of springs 11 and the centrifuge empties into aperipheral ducting (not shown).

In operation, the discs 1 and 2 rotate at a constant speed which isgenerally in the range of from 1000 to 3000 r.p.m. The preferred speedis about 1500 r.p.m. which gives a sufficiently short sedimentationtime; at this speed it is necessary to exert a force of 500 poundsweight on the top and bottom of compartments A and B to hold the twohalves of the-centrifuge together when the compartments are full. Onecompartment is filled with a completely deflocculated sample of thesuspension and will be referred to below as the reference compartment,and the other compartment is filled with a sample of the partiallydeflocculated suspension, of which it is desired to determine the degreeof deflocculation, and will be referred to below as the measurementcompartment. The centrifuging time should be substantially longer thanthe time taken to fill the sample compartment. Thus, when the fillingtime is of the order of 2 or 3 seconds, the centrifuging time should beat least 2 minutes. Of the pair of photocells mounted below each window7, one under each window is used to measure the light intensity fallingon it in terms of electrical resistance; the second photocell below thewindow of the reference compartment is for control purposes, which willbe described below; and the second photocell below the window of themeasurement compartment is a dummy included only for symmetryand'balance.

As the solids concentration of a suspension in the compartments A and Bincreases, so does the optical obscuration of the light falling on thephotocell 8. This means that if the intensity of the light directed uponthe window 6 of the reference compartment is maintained constant, theelectrical resistance of the photocells 8 below the window 7 will dependupon the initial solids concentration of the suspension. Although the(incident light intensity/electrical resistance) curves for thephotocells 8 below the windows 7 of the two compartments are similar,they are not identical so that, with a constant illumination from thering of lamps and a constant ratio for the solids concentrations of thesuspensions in compartments A and B, the ratio of photocell resistanceswill depend, to some extent, on the absolute values of the solidsconcentrations of the suspensions in the compartments. In order tominimize this effect, the brightness of the lamp is automaticallyadjusted to give a substantially constant incident light intensity onthe control photocell of the reference compartment, whatever the solidsconcentration of the suspension between the windows 6 and 7 of thatcompartment For a fixed ratio of solids concentrations of the suspensionin the two compartments the ratio:

Incident light intensity on the measurement photocell decreases as theabsolute values of the solids concentrations increase. This difficultyis overcome by varying the duration of centrifuging according to theinitial solids concentration of the suspension being examined. Prior tocommencement of centrifuging the ratio:

Solids concentration of the suspension between the windows of thereference compartment will be unity, and as the large, flocculatedparticles are centrifuged out of the suspension in the measurementcompartment, the ratio will progressively decrease, the rate of decreasedepending on the degree of deflocculation. If a suspension of highinitial solids concentration is centrifuged for a shorter time, thesolids concentration ratio will be higher and this will offset thereduction in the incident light intensity ratio. In order to accommodatethe range of suspension solids concentration encountered in practice thecentrifuging time is generally varied from about 3 to about 3 /zminutes.The variable time is obtained by means of a timing unit which introducesa variable time delay into the program during centrifuging. With thelamps at full brightness, the timing unit makes a comparison of thecontrol photocell resistance (which is dependent on the solidsconcentration of the suspension) with a resistance which decreasessteadily with time (a potentiometer driven by an electric clock). Whenthese resistances are in a predetermined ratio, a relay is operated tocontinue the program cycle. The program cycle is preferably continued atleast 8 seconds and not more than 12 seconds before the end of therequired centrifuging period. As centrifuging proceeds the solidsconcentration of the reference sample, and hence the resistance of thecontrol photocell, falls slowly with time (as larger particles arethrown out of the suspension) and the clock-control resistance morerapidly. Also a suspension with a high initial solids concentrationstarts with a high photocell resistance; and the higher the initialsolids concentration of the suspension, the shorter will be the timetaken for the clock-controlled resistance to fall to the prescribedproportion of the control photocell resistance.

The use of the apparatus of the present invention for measuring andcontrolling the degree of deflocculation of a clay suspension will nowbe described with reference to FIGS. 1, 2 and 3 of the accompanyingdrawings. In FIG. 3 of the drawings, unless otherwise designated, linesbearing single arrows represent electrical connections, lines bearingdouble arrows represent conduits, and lines bearing triple arrowsrepresent mechanical connections.

The apparatus shown schematically in FIG. 3 comprises a centrifuge whichis constructed as described above with reference to FIGS. 1 and 2, ametering unit which regulates the supply of suspensions to thecentrifuge, a program control unit which regulates the operation of thewhole apparatus, and various ancillary equipment as described below.

The apparatus is connected to a conduit 31 through which there is fed amain stream of a clay suspension. Deflocculant is injected into theconduit 31. through a pipe 32. In order to measure the degree ofdeflocculation brought about by the injection of the deflocculant, asample of the main stream is tapped off through a pipe 33.

A pump 34 feeds the sample to an on-off solenoid valve 35. The solenoidvalve 35 is opened for a sufficient period to enable the clay suspensionto flow into two metering cylinders 36 and 37 up to the level ofoverflow pipes. When the solenoid valve 35 is closed the clay suspensiontapped off through pipe 33 is reintroduced into the main stream throughpipe 33a. A pump 3% injects into the portion of the clay suspension inthe cylinder 37 a quantity of deflocculant sufficient to fullydeflocculate the portion, this portion thereby becoming the referencesample. After allowing sufficient time for uniform dispersion, meteringoverflow valves 39 and 40 are opened to allow the suspension level tofall to a lower overflow level (the reason for initially making up alarger sample than is required, is that the deflocculant dose may bemetered more accurately in larger quantifies).

A solenoid valve 41 is closed so that oil under pressure is delivered bya pump 58 to the hydraulic cylinders 10, thus bringing together the twodiscs 1 and 2 (FIG. 2) of the centrifuge to form the sealed compartmentsA and B (FIGS. 1 and 2).

Discharge valves 42 and 43 at the base of the metering cylinders 36 and37 are opened to transfer the samples into their respective samplecompartments in the centrifuge. When centrifuging hasbeen in progressfor about 1 minute the following operations are performedsimultaneously:

a. a ring of lamps 4- 1 is switched on;

b. a centrifuging time control circuit 45 is brought into operation; and

c. an electromagnetic clutch between a servomotor and a measured valuepotentiometer 50 is engaged.

After a brief interval the program control unit switches off its owndrive motor 46. Ten seconds before the completion of the centrifugingtime, as determined by the centrifuging time control circuit 45, thedrive motor 46 is switched on again. The control photocell under thewindow 7 of the reference compartment is switched into a brightnesscontrol circuit 47, and the brightness of the lamps is adjusted.

As a result of the different degrees of deflocculation of the samples inthe compartments of the centrifuge, an out-ofbalance signal from theWheatstone bridge containing the photocells of the reference andmeasurement compartments is fed to a servoamplifier 48, which controls aservomotor 49 which is turn drives the measured value potentiometer 50,through an electromagnetic clutch, in a direction tending to balance theWheatstone bridge. The following operations are then simultaneouslyperformed:

a. the electromagnetic clutch is disengaged so that the final positionof the measure value potentiometer 50 is retained and can he recorded ona measured value recorder 59;

b. the lamps 4d are switched off; and

c. a timer circuit 51 and a motor 52 are switched on.

A direction control circuit 53 senses the polarity of the differencebetween the positions of a desired value potentiometer 54 and themeasured value potentiometer 50 and selects the direction of the motor52 accordingly whereby a flow control valve 55 is operated. The timercircuit 51 determines the magnitude of the difference between the twopotentiometer positions and the motor 52 is energized for acorresponding length of time. The solenoid valve 41 is opened thusreleasing the oil pressure in the hydraulic cylinders and allowing thetwo halves of the centrifuge to separate, under the action of springs11, and eject their contents. A solenoid valve 56 is opened therebyallowing rinsing water to flow into the metering cylinders 36 and 37 andthence into the centrifuge. The solenoid valve ll is opened and closedseveral time to allow the centrifuge to fill with water to overflowingbefore discharging, whereafter the solenoid valve 56 is closed. Thesolenoid valve 41 is then opened and the centrifuge is allowed to spindry, thus completing the cycle of operations.

The various parts of the apparatus are brought into operation at theappropriate time by means of a number of cam operated switches 57 which,for example, energize the solenoid valves 35, ll and 56.

We claim:

1. An apparatus comprising a centrifuge having two separate compartmentswherein the improvement comprises each of the two separate compartmentsbeing provided with a window, whereby light can be passed throughmaterial present in each of said compartments, and a photocell arrangedso as to intercept the light passing through said window and thematerial in each of said compartments and to produce a signalcorresponding to the optical density of material present in each of saidcompartments at a fixed distance from the axis of rotation of saidcentrifuge, and further being provided with means for comparing thesignals obtained from the photocells, said photocells being positionedto rotate with said centrifuge.

2. An apparatus as claimed in claim 1, wherein the centrifuge comprisestwo cooperating disc members arranged face-to-face, at least one ofwhich is formed with two recesses which constitute the two separatecompartments of the centrifuge.

3. An apparatus as claimed in claim 2, wherein there is provided meanswhich enable the disc members to be separated whilst the centrifuge isin operation whereby the said compartments can be emptied.

4. An apparatus as claimed in claim 2, wherein there are provided twotruncated hollow cones arranged one within the other and coaxially withthe axis of rotation of the centrifuge, one truncated hollow cone beingin communication with one of said two separate compartments and theother truncated hollow cone being in communication with the other ofsaid two separate compartments, the arrangement being such that said twoseparate compartments can be filled with suspensions of fine particlesduring operation of the centrifuge.

5. An apparatus as claimed in claim l, wherein one of said two separatecompartments of the centrifuge has associated therewith a photocellwhich measures the optical density of the sample contained in said onecompartment, and wherein the other of said two separate compartments ofthe centrifuge has associated therewith two photocells, one of which twophotocells measures the optical density of the sample contained in saidother compartment and the other of which two photocells acts as acontrol photocell which controls means whereby the amount of lightincident on said two photocells can be adjusted to a substantiallyconstant value which is independent of the original solids concentrationof the sample contained in said other compartment.

6. An apparatus comprising a centrifuge having two separate compartmentswherein the improvement comprises:

a. said two separate compartments including two cooperating disc membersarranged face-to-face, at least one of which if formed with recesseswhich constitute the separate compartments of the centrifuge;

b. means to enable the disc members to be separated whilst thecentrifuge is in operation whereby said compartments can be emptied;

c. each of said two separate compartments being provided with a window,whereby light can be passed through material present in saidcompartments, and a photocell arranged so as to intercept the lightpassing through said window and the material in said compartment and toproduce a signal corresponding to the optical density of materialpresent in said compartment at a fixed distance from the axis ofrotation of said centrifuge;

d. means for comparing the signals obtained from the photocells; and

e. two truncated hollow cones arranged one within the other andcoaxially with the axis of rotation of the centrifuge, one truncatedhollow cone being in communication with one of said separatecompartments and the other truncated hollow cone being in communicationwith the other of said compartments, the arrangement being such thatsaid compartments can be filled with suspensions of fine particlesduring operation of the centrifuge.

7. An apparatus as claimed in claim 6, wherein the means which enablethe disc members to be separated whilst the centrifuge is in operationcomprise spring means which is held under compression, in cavitiesformed in the two disc members, by the action of a plurality ofhydraulic cylinders each containing a piston tending to urge the twodisc members together when the two disc members are to be held together,and which spring means is allowed to return to its uncompressed state,thereby separating the two disc members, by the release of the hydraulicpressure in said hydraulic cylinders when ever the two disc members areto be separated.

8. A process for measuring the degree of deflocculation of a suspensionof fine particles which process comprises separating said suspensioninto two portions, adding to one of said two portions a quantity ofdeflocculant sufficient to completely deflocculate said one portion,introducing the portions of said suspension one into each of twoseparate compartments of a centrifuge, said two compartments each beingprovided with a window, whereby light can be passed through the portionof said suspension therein, and each having associated therewith aphotocell arranged so as to interrupt the light passing through saidwindow and the suspension in said compartments and to produce a signalcorresponding to the optical density of the portion of said suspensiontherein at a fixed distance from the axis of rotation of saidcentrifuge, centrifuging said two portions for a time dependent on theinitial solids concentration of the suspension, passing light throughsaid windows and through said two portions of the suspension, andmeasuring the optical densities of the two portions at said fixeddistance from the axis of rotation of said centrifuge, whereby thedegree of deflocculation of the original suspension can be determined.

9. A process as claimed in claim 8, wherein the intensity of the lightpassing through said windows is adjusted to give a substantiallyconstant incident light intensity on said photocells whatever theinitial solids concentration of the portion of the suspension which iscompletely detlocculated.

2. An apparatus as claimed in claim 1, wherein the centrifuge comprisestwo cooperating disc members arranged face-to-face, at least one ofwhich is formed with two recesses which constitute the two separatecompartments of the centrifuge.
 3. An apparatus as claimed in claim 2,wherein there is provided means which enable the disc members to beseparated whilst the centrifuge is in operation whereby the saidcompartments can be emptied.
 4. An apparatus as claimed in claim 2,wherein there are provided two truncated hollow cones arranged onewithin the other and coaxially with the axis of rotation of thecentrifuge, one truncated hollow cone being in communication with one ofsaid two separate compartments and the other truncated hollow cone beingin communication with the other of said two separate compartments, thearrangement being such that said two separate compartments can be filledwith suspensions of fine particles during operation of the centrifuge.5. An apparatus as claimed in claim 1, wherein one of said two separatecompartments of the centrifuge has associated therewith a photocellwhich measures the optical density of the sample contained in said onecompartment, and wherein the other of said two separate compartments ofthe centrifuge has associated therewith two photocells, one of which twophotocells measures the optical density of the sample contained in saidother compartment and the other of which two photocells acts as a''''control photocell'''' which controls means whereby the amount oflight incident on said two photocells can be adjusted to a substantiallyconstant value which is independent of the original solids concentrationof the sample contained in said other compartment.
 6. An apparatuscomprising a centrifuge having two separate compartments wherein theimprovement comprises: a. said two separate compartments including twocooperating disc members arranged face-to-face, at least one of which ifformed with recesses which constitute the separate compartments of thecentrifuge; b. means to enable the disc members to be separated whilstthe centrifuge is in operation whereby said compartments can be emptied;c. each of said two separate compartments being provided with a window,whereby light can be passed through material present in saidcompartments, and a photocell arranged so as to intercept the lightpassing through said window and the material in said compartment and toproduce a signal corresponding to the optical density of materialpresent in said compartment at a fixed distance from the axis ofrotation of said centrifuge; d. means for comparing the signals obtainedfrom the photocells; and e. two truncated hollow cones arranged onewithin the other and coaxially with the axis of rotation of thecentrifuge, one truncated hollow cone being in communication with one ofsaid separate compartments and the other truncated hollow cone being incommunication with the other of said compartments, the arrangement beingsuch that said compartments can be filled with suspensions of fineparticles during operation of the centrifuge.
 7. An apparatus as claimedin claim 6, wherein the means which enable the disc members to beseparated whilst the centrifuge is in operation comprise spring meanswhich is held under compression, in cavities formed in the two discmembers, by the action of a plurality of hydraulic cylinders eachcontaining a piston tending to urge the two disc members together whenthe two disc members are to be held together, and which spring means isallowed to return to its uncompressed state, thereby separating the twodisc members, by the release of the hydraulic pressure in said hydrauliccylinders when ever the two disc members are to be separated.
 8. Aprocess for measuring the degree of deflocculation of a suspension offine particles which process comprises separating said suspension intotwo portions, adding to one of said two portions a quantity ofdeflocculant sufficient to completely deflocculate said one portion,introducing the portions of said suspension one into each of twoseparate compartments of a centrifuge, said two compartments each beingprovided with a window, whereby light can be passed through the portionof said suspension therein, and each having associated therewith aphotocell arranged so as to interrupt the light passing through saidwindow and the suspension in said compartments and to produce a signalcorresponding to the optical density of the portion of said suspensiontherein at a fixed distance from the axis of rotation of saidcentrifuge, centrifuging said two portions for a time dependent on theinitial solids concentration of the suspension, passing light throughsaid windows and through said two portions of the suspension, andmeasuring the optical densities of the two portions at said fixeddistance from the axis of rotation of said centrifuge, whereby thedegree of deflocculation of the original suspension can be determined.9. A process as claimed in claim 8, wherein the intensity of the lightpassing through said windows is adjusted to give a substantiallyconstant incident light intensity on said photocells whatever theinitial solids concentration of the portion of the suspension which iscompletely deflocculated.